Starling Forces: The Key to Understanding Fluid Balance

Starling Forces are fundamental principles that regulate the passive movement of water between the tiny blood vessels known as capillaries and the surrounding interstitial fluid, which is the fluid that bathes the cells in our body. These forces play a crucial role in maintaining the balance of fluids in our body.

The direction and rate of water movement between the capillary and interstitial compartments are determined by these forces. In other words, Starling Forces dictate whether water will move from the capillaries into the interstitial space, or vice versa, and how quickly this exchange will take place.

These forces are influenced by the balance between hydrostatic and oncotic pressures. Hydrostatic pressure, which is the pressure exerted by the fluid on the walls of the blood vessels, tends to push water out of the capillaries. On the other hand, oncotic pressure, which is primarily determined by the concentration of proteins in the blood, tends to pull water back into the capillaries.

Key Variables

The direction of water exchange between the plasma and interstitial fluid across the capillary wall is largely determined by a combination of the relative hydrostatic and oncotic pressures of these two compartments. The rate of exchange is governed by the permeability of the capillary wall itself.

• Hydrostatic Pressure Gradient

Hydrostatic Pressure refers to the physical force of fluids against their enclosing barriers. Plasma within capillaries has a positive hydrostatic pressure, a remnant of the blood pressure generated by the heart. Fluid within the interstitial place generally has a negative hydrostatic pressure, likely due to the action of lymphatic pumping.

• Oncotic Pressure Gradient

Oncotic Pressure refers to the osmotic pressure generated by the presence of proteinacious solutes. Because plasma proteins cannot cross the capillary barrier, these osmotically-active solutes are at higher concentration in the plasma than in the interstitial fluid. Consequently, the oncotic pressure within the plasma is higher than the oncotic pressure within the interstitial fluid, generating an oncotic pressure gradient between these two compartments.

• Vascular Permeability

The histological architecture of capillaries determine the permeability of capillaries to water and this can vary by over two orders of magnitude in different capillary beds. For example, the fenestrated architecture of the glomerular capillaries causes these vessels to display an extremely high permeability to water whereas the extremely tight architecture of the blood brain barrier results in an extremely low water permeability. It should be also pointed out that acute inflammation or certain types of damage to the capillary wall such as during burns can increase the water permeability of the microcirculation.

Formal Relationship

• Jv = Kf [(Pc-Pi)-(Πc – Πi)]
  • Jv = Net fluid movement (ml/min). A positive value indicates movement out of the circulation.
  • Kf = Vascular Permeability Coefficient
  • Pc = Capillary hydrostatic pressure
  • Pi = Interstitial hydrostatic pressure
  • Πc = Capillary oncotic pressure
  • Πi = Interstitial oncotic pressure

Starling Forces in Physiology

 

For most capillary beds the direction of starling forces reverses as blood moves through the capillary bed. Near the arteriolar side of the microcirculation, starling forces result in outward fluid filtration whereas toward the venous side of the microcirculation starling forces result in fluid resorption. This is a result of changes in the hydrostatic and oncotic pressures of the plasma fluid as blood moves through the microcirculation. Importantly, the hydrostatic and oncotic pressures of the interstitial fluid largely remain constant.

• Arterial Side

Capillary Hydrostatic Pressure (Pc) is at its highest value nearest to the high-pressure arteries, whereas the interstitial hydrostatic pressure (Pi) is at its constant negative value. At the arterial side of the microcirculation, the plasma oncotic pressure is relatively high whereas the interstitial oncotic pressure is always low if not absent. Consequently, the hydrostatic pressure gradient highly favors outward filtration of water whereas the oncotic pressure gradient favors inward resorption of water. At the arterial side, the outward hydrostatic pressure gradient is larger than the inward oncotic pressure gradient and thus net fluid filtration is in the outward direction.

• Venous Side

Toward the venous side of the microcirculation, the capillary hydrostatic pressure (Pc) has declined due to the resistance to blood flow generated by the capillary. Furthermore, progressive outward filtration of water as blood travels through the microcirculation causes plasma proteins to become concentrated within the circulation, thus raising the plasma oncotic pressure. Because the outward hydrostatic pressure gradient has declined and the inward oncotic pressure gradient has increased, there is now net fluid resorption.

• Net Filtration

Outward fluid filtration on the arterial side of the microcirculation largely balances inward fluid filtration on the venous side of the microcirculation. When added together there is a minuscule net outward filtration of fluid from most capillary beds. When this outward filtration is summed across all tissue capillary beds there appears to be only a few milliliters of outward fluid filtration per minute throughout the entire body. Although small, this net outward fluid filtration must be ultimately returned to the circulation to avoid long-term increases in interstitial fluid volume and long-term declines in circulatory volume. Return of filtered fluid is ultimately achieved by lymphatic vessels as discussed in Lymphatic Physiology.

Starling Forces in Pathology

A number of diverse pathological processes can derange the finely balanced fluid filtration of capillary beds by changing the key variables of the Starling Forces. In most cases this results in excessive water filtration out of the capillaries. When this occurs locally it can result in localized edema whereas if it occurs globally throughout the body’s microcirculation, this can result in generalized edema. We provide a few examples below for didactic purposes.

• Derangements of Vascular Permeability

Derangements of vascular permeability occur when the tight architecture of the capillaries is damaged. This can occur due to immune-mediated processes in acute inflammation or due to thermal damage in burns.

• Derangements of Hydrostatic Pressure Gradient

Derangements of the hydrostatic pressure gradient usually occur due to pathologically increased hydrostatic pressure on the venous side of the microcirculation. This usually occurs due to ineffective venous drainage of blood, causing backup of blood and thus increased hydrostatic pressure on the venous side. This is termed congestion and can occur due to venous thrombosis or right heart failure to name a few examples.

• Derangements of Oncotic Pressure

Derangements of the oncotic pressure gradient usually occur due to reductions in plasma oncotic pressure from poor synthesis or excessive loss of plasma proteins, especially albumin. Reduced plasma oncotic pressure in turn reduces in the inward oncotic pressure gradient and thus allows for increased outward fluid filtration. Reduced plasma protein synthesis, especially that of albumin, can occur in a variety of pathologies including protein-energy malnutrition, cirrhosis, and nephrotic syndrome.

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